As an organic EL panel provided with organic EL devices serving as constant current drive devices, there is, for example, one described in JP-A-2001-142432. This is an organic EL panel of a dot matrix type in which plural anode electrode lines using a conductive transparent film such as an ITO (Indium Tin Oxide) are formed in parallel with each other on a translucent insulating support substrate such as a glass substrate, an organic layer (organic EL layer) is formed on the back of these anode electrode lines, plural parallel cathode electrode lines using a metal evaporated film such as aluminum is formed on the back of this organic layer so as to be perpendicular to the anode electrode lines, and the organic layer is held by these anode electrode lines and cathode electrode lines. The organic EL panel has been attracting attentions as a display, which is capable of realizing low power consumption, high display quality, and reduced thickness, substituting a liquid crystal display.
As a drive circuit for such an organic EL panel, there is one shown in FIG. 6. Such a drive circuit includes an organic EL panel 1, a cathode side drive circuit 2, an anode side drive circuit 3, and a control unit 4.
The organic EL panel 1 is formed by disposing organic EL devices E11 to Enm bearing pixels in a lattice shape. In a structure of these organic EL devices E11 to Enm, an organic layer including at least a light-emitting layer is held in crossing parts of plural anode electrode lines 1A, which are provided so as to be laid along a vertical direction, and plural cathode electrode lines 1B, which are provided so as to be perpendicular to the anode electrode lines 1A. If represented as an equivalent circuit, the organic EL devices E11 to Enm are formed with one ends thereof connected to the anode electrode lines 1A (anode side of a diode component) and the other ends connected to the cathode electrode lines 1B (cathode side of a diode component).
The cathode side drive circuit 2 is provided with plural scanning switches 2a1 to 2am corresponding to the respective cathode electrode lines 1B and selects a reverse bias voltage Vb, which becomes a power supply voltage on the cathode side in the respective organic EL devices E11 to Enm, or a ground potential (0V) with the scanning switches 2a1 to 2am based upon a control signal of the control unit 4. That is, the organic EL devices E11 to Enm come into a non-light emitting state when the reverse bias voltage Vb is selected by the scanning switches 2a1 to 2am and come into a light emitting state when the ground potential is selected by the scanning switches 2a1 to 2am. 
The anode side drive circuit 3 is provided with constant current sources 3a1 to 3an, which supply a constant current (drive current) to the anode electrode lines 1A, respectively, in association with them, and is constituted such that the constant current from these constant current sources 3a1 to 3an is supplied to the respective anode electrode lines 1A via the respective drive switches 3b1 to 3bn. Changeover of the respective drive switches 3b1 to 3bn is determined based upon a control signal from the control unit 4.
The control unit 4 includes a microcomputer and, for example, when travel information of a vehicle is inputted from various sensors, in an attempt to perform predetermined arithmetic operation processing and to display various kinds of information such as a vehicle speed, an engine speed, and residual fuel on the organic EL panel 1, outputs the travel information to the cathode side drive circuit 2 and the anode side drive circuit 3, respectively, as a control signal, and selectively turns ON/OFF the scanning switches 2a1 to 2am and the drive switches 3b1 to 3bn corresponding to the cathode electrode and anode electrode lines 1B, 1A necessary for causing the organic EL devices E11 to Enm to emit light, thereby causing the organic EL panel 1 to display predetermined information. The drive circuit of the organic EL panel comprises the above portions.
In such a drive circuit of the organic EL panel 1, gradation control is performed which is based upon pulse width modulation (PWM) of the cathode and anode scanning lines 1B, 1A corresponding to the scanning switches 2a1 to 2am and the drive switches 3b1 to 3bn in the cathode side drive circuit 2 and the anode side drive circuit 3, and the organic EL devices E11 to Enm bearing pixels are driven by the reverse bias voltage (output voltage) Vb, which is a non-selected/selected voltage in the cathode side drive circuit 2, and an output current from the constant current sources 3a1 to 3an in the anode side drive circuit 3.
However, in the organic EL devices E11 to Enm which have temperature dependency making it possible to emit light with a smaller drive voltage as temperature rises, in order to eliminate reactive power consumed in the anode side drive circuit 3, the organic EL devices E11 to Enm have to be controlled such that a drive voltage is reduced as an ambient temperature rises and that the drive voltage is increased as the ambient temperature falls.
In addition, there is a problem as described below. If the reverse bias voltage Vb in the cathode side drive circuit 2 suitable for the ambient temperature is not given to the organic EL devices E11 to Enm, in gradation control for one scanning line (light intensity control for one period based upon PWM) in the organic EL device E11 to Enm emitting light by the reverse bias voltage (output voltage) Vb and the output voltage of the constant current sources 3a1 to 3an, the reverse bias voltage Vb on the cathode side becomes larger than a light emission start voltage (drive voltage of an organic EL device suitable for an ambient temperature) in the organic EL devices E11 to Enm. When the reverse bias voltage Vb is selected by the scanning switches 2a1 to 2am in the cathode side drive circuit 2 in this state, in an organic EL device coupled to the selected cathode electrode line 1B, a charging current is generated by a capacitor component included in the organic EL device. Thus, the reverse bias voltage Vb reaches a light emission voltage concurrently with sharp rising, and light exceeding a predetermined luminance is emitted, although this occurs only in an instance. Note that, although influence of the light emission luminance exceeding the predetermined luminance in the organic EL devices E11 to Enm is relatively inconspicuous if a current application time from the constant current sources 3a1 to 3an by the gradation control is long, the influence becomes more conspicuous as the current application time is shortened by the gradation control.
The present invention has been devised in view of the above-mentioned problem and provides a drive circuit for an organic EL panel capable of controlling generation of reactive power even in the case in which an ambient temperature changes and, at the same time, keeping a light emission luminance of an organic EL device bearing pixels constant.